The corneal topography of an eye involves a micrometer-accurate examination method of the cornea, in which some kind of a map of the surface of the cornea is created. The ophthalmologist measures the extent of the curvature of the cornea at thousands of single points. The corneal topography provides an exact image of the curvature of the cornea. Based on these results, the ophthalmologist can determine possible pathological changes. The exact measurement of the corneal topography is also of great importance for correcting refractive errors.
In view of newer applications, such as                cataract surgery,        IOL determination,        contact lens adjustment, and        refractive laser surgery,topographic procedures face special challenges regarding accuracy and reproducibility of the measurements, which make it desirable to improve the traditional topographic procedures or develop new methods.        
The currently known topographic procedures are based on specially adapted methods of deflectometry, fringe projection and triangulation. For example, it is possible to determine accurately the corneal radii up to app. +/−0.05 mm, using a keratometer or an ophthalmometer.
The term deflectometry involves the contactless capture or measurement of reflective surfaces, in which technologies from photometry or radiometry, photogrammetry, laser scanning or laser distance measurement are used.
The keratometer is an instrument for measuring the surface curvature of the cornea of an eye and for determining the corneal progression. In the process, an illuminated object is placed at a known distance and the reflection of the cornea is measured to be able to draw conclusions about the curvature of the cornea.
Two further traditional topographic procedures are based on the projection of a placido-ring system or the use of a Scheimpflug camera.
In the placido-ring-based method, a system of alternating black and light rings is projected at regular intervals on the anterior corneal surface. By evaluating the reflection of the ring system on the cornea, the anterior corneal surface can be reconstructed visually, and its curvature can be measured.
In the Scheimpflug method, on the other hand, pictures are taken with the camera from different viewing directions and these pictures are used to determine the shape of the anterior and posterior surface of the cornea.
With these two topography devices, elevation data can be collected with an accuracy of app. 1 μm.
Newer topography methods include the optical coherence tomography (OCT), which currently allows for a low resolution in the range of app. 10 μm, which can be increased in the future in the accuracy range of less μm. However, compared to traditional topography methods, these systems are very expensive.
The basic principle of the OCT method is based on white-light interferometry and compares the duration of a signal by application of an interferometer (commonly a Michelson Interferometer). For this purpose, an arm with known optical path length (=reference arm) is used as reference for the measuring arm. The interference of the signals of both arms results in a pattern from which the relative optical path length within an A scan (single low signal) can be read. In the one-dimensional screening methods, the beam is guided transversally in one or two directions, analogous to ultrasound technology, whereby it is possible to record a flat B-scan or a three-dimensional tomogram (C-scan). For example, for one A-scan comprising 100 single A-scans one second of measuring time is required.
The measurement resolution of the OCT method is determined by the so-called coherence length of the light source used which is approximately 15 μm. Because of its special aptitude for examining optically transparent media, the method is widely used in ophthalmology.
In OCT methods used in ophthalmology, two different types have been established. To determine the measuring values, in the first type, the length of the reference arm was changed, and the intensity of the interference was continually measured, without taking the range into consideration. This method is described as “time domain” method. However, in the other method, which is described as “frequency domain”, the range is considered for determining the measuring values and the interference of the single spectral components is captured. Therefore, one is called a signal in the time domain, and the other a signal in the frequency domain.
In current prior art, different types of work have become known, which use OCT data for determining the topography of the cornea of an eye and compare them with conventional methods.
U.S. Pat. No. 8,770,753 B2 discloses multiple radial and circular OCT scan patterns centered on the cornea and the anterior eye segment to determine with these data, among other things, the topography and pachymetry of the cornea. For a long time the analysis of data of the cornea (anterior and posterior surface) has been examined in research on the basis of OCT methods, while still facing different challenges:                1. OCT imaging usually has an adequate axial depth resolution but a limited lateral resolution. This often results from poor image reproduction of the corneal surface.        2. The spatial positions of the cornea, in which information from the OCT scan is available depend on the design of the scan pattern. For example, it is desirable that denser scans are produced over critical areas of the cornea, and less scans are required in the more continuous regions.        3. To be able to determine an accurate topography of the cornea, repeated and accurate monitoring and control of the scanners are necessary.        4. In order to have accurate synchronization between the OCT scan data and the dynamic of eye fixation or ocular movement, an additional quick eye tracker is required.        5. Alternatively, to solve the problem under 4), an ultra-highspeed OCT scanner is necessary, which can collect the complete data record in the ms range analogous to the camera systems. Only in this case would ocular movement not play any part. Currently, such scanner systems are not available.        6. As a further alternative solution to 4), a robust and accurate motion detection could be determined from the OCT data, in order to correct the resulting topographic data according to the movements.        
In [1], Sergio Ortiz et al. collect corneal topographic data by use of a spectral domain OCT (s-OCT). There, distortions based on OCT scan geometry are compensated, and the problems of ocular movement during the scanning period are discussed. The OCT topographic data collected by use of test objects and patient data are compared with placido data and Scheimpflug data of conventional corneal topography and evaluations are presented. It is assumed that OCT topography is feasible, but there are still some problems to be solved.
In [2], Karol Karnowski et al. collect and examine in a clinical study topographic data with swept source OCT. Because of the high speed of the ss-OCT method, it was already possible to collect within the time period <0.25 s a dense data network for corneal topography and compare said data with the conventional placido and Scheimpflug data for topography. The axial resolution of the ss-OCT system important for collecting elevation data of the cornea amounted to only 20 μm. Considering that in a corrective refraction of the cornea by application of LASIK, in an optical zone of 5.5 mm, centrally only approximately 10 μm tissue is removed for a dioptric correction, it would only be possible to achieve an accuracy of 2 D, which is unacceptable and demonstrates the current limitations of the prior art.
In [3], among other things, the advantages of using OCT-based topography are discussed in comparison with conventional keratometry and topography. In this context, the complete representation of the cornea by application of OCT is evaluated as being very important, especially when the cornea has been changed by refractive laser surgery. To be able to control the problem of ocular movement artefacts with comparatively slow OCT, optimized scan strategies are specified, which allow for an improved evaluation of the scan data. By means of correction procedures with regard to optical distortions of the OTC, it was possible to achieve a correlation of the topographic data of 0.1+/−0.53 D collected in comparison to the simulated keratometry from topographic data.
In [4], the combination of placido-topographic data and biometric data, which were collected by means of OCT for eye length, were discussed by T. Oltrup, et al. The aim is to open up new possibilities for determining artificial intraocular lenses (IOL). Here, the topographic data are determined in conventional manner, and, when using the IOLMaster 500, an A scan is used for length measurements of the eyes.
Prior art shows that efforts are made to use the OCT methods also for collecting topographic data of the cornea. Based on scan geometries provided for the OCT, correction procedures are required to be able to specify the real corneal topography. Through measurements on a reference object, it was possible to evaluate in principle and show the validity of the correction for a spherical shape ([2]).
Compared to conventional methods of keratometry and placido topography, which can collect a data set without distracting eye movements by means of camera chip exposure, the relatively long scan period of the OCT method has a disadvantage and requires further correction procedures.
The axial resolution of the OCT methods is limited by the spectral scanning width of the laser source. With the high-resolution OCT of approximately 50 nm, it is possible to obtain approximately 5-6 μm. With the ultra-high-resolution OCT, for example on the basis of complex fs lasers, it is possible to obtain app. 1-3 μm with approximately 1000 nm.
There is a deficiency in prior art in that it is currently not possible to provide topographic data of the cornea with currently available cost-effective OCT systems. In addition to resolution and reproducibility, the long scanning period in connection with the ocular movement is a problem that can currently only be controlled with additional eye trackers.
Combination devices having a placido-ring projection and an OCT device would require a comparatively large installation space, wherein the placido disc particularly limits the operator's view on an eye pair and results in time-consuming processes. When a large diameter is to be achieved in the topography of up to app. 16 mm all the way to the area of the sclera, the placido projector would require correspondingly larger diameters and would further complicate the handling process, or there would be a very limited implementation for the optical design.
A further disadvantage of the conventional placido topography and even keratometry involves the lack of measuring data in the central optical zone of app. 2 mm-0.8 mm diameter, because these would be used for data acquisition with the measuring camera.
Furthermore, a tear film outline during deflectometric recordings results in an internal distortion of the measuring point on the camera and complicates the process of finding the focus and reduces the measuring accuracy.
The new IOLMaster® 700 by Zeiss [5] already collects B scans for measuring the biometric axial distances in an eye and thus also a data set for surface topography of the cornea. This system, in which a visual examination of the obtained biometric data based on OCT images takes place, is characterized by better refractive results with high repeatability and clinical databank connection. In addition, the collection of OCT data is feasible with a comparatively low standard deviation of reproducibility. The following measuring accuracies can be obtained with this system:                Central corneal thickness: +/−2 μm        Anterior chamber depth: +/−11 μm                    However, to this system with a comparatively slow scan rate, the above-mentioned current disadvantages can be applied.LITERATURE                            [1] Ortiz, Sergio, et al.; “Corneal topography from spectral optical coherence tomography (sOCT)”, Biomedical Optical Express, vol. 2, no. 12, 2011, 3232-3247    [2] Karnowski, Karol, et al.; “Corneal topography with high-speed swept source OCT in clinical examination”, Biomediacal Optical Express, vol. 2, no. 9, 2011, 2709-2720    [3] Izatt, Joseph A., et al.; “Expanding the use of OCT”, Optics & Photonics News, April 2014, 34-41    [4] Oltrup, T., et al.; “Placido-Hornhauttopographie kombiniert mi optischer Biometrie—erste Egebnisse”; Klinische Monatsblaetter der Augenheilkunde 2013; 230, 519-523, published in Germany CZ-January 2015; ©Carl Zeiss Meditec AG, 2014.